In thermography, nightscopes, security systems, and the like, imaging systems for electromagnetic waves of the wavelength range from several to several tens of micrometers are used. The electromagnetic waves of the above-described wavelength range are referred to as far-infrared rays. In imaging systems that image far-infrared rays, data of far-infrared rays are captured by an imaging element in which infrared detecting elements are arranged in array. Conventional infrared detecting elements include a thermopile type which utilizes electromotive force generated by a temperature change caused by collected infrared rays, a bolometer type which utilizes a change in electric resistance generated by a temperature change, a pyroelectric type which utilizes a change in an amount of charge generated by a temperature change, a photon detecting type which utilizes a energy level transition of an electron caused by absorption of a photon, and the like. When any type of infrared detecting element is used, incoming infrared rays must be collected and directed onto the imaging element by optics of the constituent imaging system.
Conventionally, glass and synthetic resin are used as materials of optical lenses. In particular, synthetic resin is advantageous in that optical lenses can be made from it at lower costs using injection molding or the like. In general, however, transmittance of a synthetic resin material for far-infrared rays is lower than transmittance of the synthetic resin material for visible light, and therefore a synthetic resin lens which used for visible light imaging systems typically cannot be used for far-infrared imaging systems. In other words, if a synthetic resin lens that is used for visible light imaging systems is coopted for use with far-infrared imaging systems, the amount of infrared radiation becomes insufficient because of absorption of far-infrared rays of the synthetic resin lens.
Accordingly, lenses of conventional far-infrared imaging systems typically employ relatively expensive materials that absorb less infrared radiation, such as germanium and various chalcogenides, or other materials that can only be shaped by machining and therefore require higher manufacturing costs, such as silicon (e.g., see JP2002-014283 and JP11-326757, where are hereby incorporated by reference in their entirety).
Thus, there is a need for a far-infrared imaging system including a synthetic resin lens that is capable of providing a sufficient amount of infrared radiation to an imaging element of the imaging system.